Energy accumulator arrangements for implants are known, wherein such arrangements include a battery, a capacitor, and a charging device, wherein the charging device is designed to supply an electric charge from the battery to the capacitor in accordance with a charging program. Such devices are also referred to as devices for battery and/or capacitor reformation. Contrary to the simple charging of the capacitor or battery, reforming denotes a controlled charging of the capacitor or battery, where the charging is carried out according to a charging program. Reforming a capacitor instead of uncontrolled charging is necessary in order to achieve as long a service life of the capacitor as possible.
On an electrolyte capacitor, which is used frequently in implants, a very thin oxide layer is provided on the positive electrode as an insulator between the two electrodes. A small leakage current is required between the two electrodes in order to maintain this insulating oxide layer. When this leakage current is absent, for example because the capacitor has not been charged for an extended period of time, the oxide layer substantially loses its insulating effect. As a result, very high currents may develop during the uncontrolled charging of the capacitor, which apply high loads on the capacitor and may irreversibly destroy it.
In order to achieve a long service life, capacitors of implants are therefore charged (reformed) in accordance with a charging program, which is generally designed such that a small charging current is applied to the capacitor, allowing the oxide layer to reform and regain its insulating properties. Such a reformation may take several seconds to hours and is carried out periodically (every 30 days, for example).
Methods for the controlled charging of a capacitor for an implantable device are known, for example, from U.S. Pat. No. 6,096,062. In this method, the high voltage capacitor is charged to an initial voltage which is lower than the peak voltage of the capacitor. Thereafter, a leakage current is measured. If the leakage current drops below a predetermined threshold value, the high voltage capacitor is not charged further. On the other hand, if the leakage current exceeds the predetermined threshold value, the high voltage capacitor continues to be charged up to the peak voltage and the peak voltage is maintained, allowing the high voltage capacitor to be reformed.
The problem with known maintenance systems is that the batteries and capacitors of the implants continue to have a comparatively short service life.